Target size analysis, using high energy electron radiation, was employed to estimate the functional target sizes of the adenylate cyclase systems in plasma membranes of liver, adipocytes, and turkey erythrocytes. The functional sizes of both liver and adipocyte adenylate cyclase increased with complexity of regulation, suggesting that the regulatory (hormone receptors and GTP-regulatory proteins) become linked to the catalytic unit when put in their activated states by hormones, guanine nucleotides, and fluoride ion. Prior to "coupling" the regulatory units are "multimers" of the units attached to the catalytic unit (the activated holoenzyme). By contrast, multimeric regulatory units were not detectable in turkey erythrocyte membranes; this system seems to be pre-assembled as the holoenzyme even prior to activation by catecholamines and guanine nucleotides. The regulatory units responsible for inhibition of the adipocyte cyclase system by GTP and adenosine yield a target size prior to coupling that is considerably larger than the multimeric "activating" units, thus supporting previous findings that inhibition and activation of adenylate cyclase by GTP are carried out through separate molecular structures.